Steamers in current use, such as cooking steamers, convection ovens, cooking steam warmers, steamers for defrosting frozen food, steamers for processing tea leaves, steam baths for household use, steamers for cleaning, and steamers used in restaurants and hotels, are widely used as equipment for utilizing the steam they generate.
Generally, fossil fuels (gas, petroleum, crude petroleum, coal and so forth) are burned as heat sources for large steamers in current use. This heating method, however, is not economical for compact steamers.
Relatively compact steamers in current use commonly employ electrical resistance heaters as a heat source. Such steamers obtain steam intermittently by spraying water on an iron plate which has been heated in advance with a heater or the heater's protecting tube from inside or beneath the plate. Another method involves the use of a vertical electromagnetic induction heater, which has been disclosed by this inventor (Japanese Patent Application Laid-Open No. 291,694/1990 and EPC No. 0380030A1).
However, the steamers using electrical resistance heaters as a heat source cannot be used continuously. A problem originates with the structure of the electrical resistance heater: the circumference of resistance heaters such as nichrome resistance wire heaters (the heat source) are filled with an insulator such as magnesium oxide, and the outside of the heater is surrounded by a protecting tube; therefore, heat from the heat source is indirectly promoted, and the heat cannot be promptly supplied to thermal exchanges occuring outside the protecting tube.
A further problem with the steamers utilizing electrical resistance heaters is that they cannot be used for long periods. Because of the indirect heating method, when spraying water on an iron plate after heating the protecting tube throughly, the temperature difference (.DELTA.T) between the surface of the protecting tube and evaporating surface (which is 100.degree. C. under normal pressure) can be as high as several hundred degrees; therefore, mineral elements in water, mainly calcium, are likely to stick to the evaporating surface as scale. Scale lowers the coefficient of heat transmission tremendously and leads to the enhancement of the .DELTA.T. Thus, a problem exists in that resistance heaters such as nichrome resistance wire heaters are eventually burned off. Therefore, an electrical resistance heater is not suitable for compact or ultra-compact steamers.
Additionally, the vertical electromagnetic induction heater (Japanese Patent Application Laid-Open No. 291,694/1990 and EPC No. 0380030A1) presents a problem in that the heater, which comprises at least six electromagnetic induction coils, is not appropriate for compact or ultra-compact steamers, although it is excellent for medium-sized steamers and is widely used.